Particle size effect on the crystal structure of Y2O3 particles formed in a flame aerosol process

Y2O3 is a material with wide applications. Its cubic and monoclinic polymorphs have significantly different mechanical and optical properties. The flame aerosol technique offers unique advantages in the synthesis of Y2O3 particles, but it also faces the challenge of phase control for Y2O3. One possible strategy for Y2O3 phase control is to utilize the particle size effect that causes the formation of the monoclinic phase at atmospheric pressure. In this study, the particle size effect on crystal structure was experimentally investigated for Y2O3 particles synthesized in a high-temperature flame aerosol process. A critical particle diameter of approximately 1.5 mu m was found by single-particle electron diffraction. At the critical diameter, the probability is 50% for a particle to be either cubic or monoclinic. Particles significantly smaller than the critical diameter were all monoclinic, while those significantly larger were all cubic. The critical diameter was interpreted using an analysis that involves the free energy of a particle, which includes the bulk Gibbs free energy and the surface energy.